This invention relates to beam or batch bonded crossovers utilizing supplementary insulating layers, and a method of forming beam crossovers in direct contact with such layers.
Conductive crossovers are formed between pairs of conductive paths lying on a substrate in order to bridge over intervening conductive paths, and thereby allow the design and realization of complex hybrid integrated circuits.
The preparation of a conventional beam type crossover structure requires typically a total of 20 processing steps: four materials depositions, 13 chemical steps, and three mechanical assembly steps (including bonding of integrated circuit chips and lead frames, and encapsulation). After cleaning of the substrate, the bottom conductor pattern is generated and a supplementary insulating layer is then screened over the conductor to be crossed and cured. A spacing layer of successive layers of Ti (500 Angstroms) and Cu (250,000 Angstroms) is deposited over the insulating layer. A layer of photoresist is applied over the spacing layer and the regions for the pillars are exposed, developed, and etched into the Ti and Cu. At this point, the region of the crossover span is exposed and developed in the photoresist layer, and pillars and span are formed by plating gold in the exposed areas. Finally, the photoresist is removed and the copper spacing layer is preferentially etched out. The completion of the circuit includes bonding of integrated circuit chips and of lead frames or connectors, and finally the encapsulation and testing. Thus, it may be seen that crossover production requires many stages of processing followed by much handling.
Use of the supplementary insulating layer over the conductor to be crossed results in higher initial yields and less sensitivity to handling damage. A typical prior art insulating layer would comprise approximately 85% by weight of a silicone resin, such as Dow Corning's DC-648, in a solvent, such as xylene, and an addition of 15% by weight of fumed silica to make the mixture screenable without runout. Such prior art resin insulating layers, while having good moisture resistance, soften, swell, and leach with solvents used during the processing of the circuits. In addition, curing of these silicone layers causes undesirable evaporation and redeposition of volatile resin components. A further problem is that the difference in the thermal expansion coefficient of the dielectric and the ceramic with its thin metallizations can lead to poor adhesion of the insulating layer to the substrate and conductor patterns if the circuit is submitted to changes in temperatures during the various processing steps.
It is therefore an objective of this invention to produce beam or batch bonded crossovers utilizing an insulating layer with reduced susceptibility to solvents, minimum volatility, and adequate moisture resistance.
A further problem is that the high curing temperatures required for some insulating layers can cause interdiffusion of metals in temperature sensitive conductor systems and that the curing times are inconveniently long.
Another objective of this invention is therefore to produce crossovers utilizing a low temperature curing insulating layer with improved screening properties, an expansion coefficient matched to the substrate, and maximum long term chemical and mechanical stability.
Yet another object of this invention is to simplify and improve the processing sequence of the crossover structure.
Another problem with prior art crossovers is that they add capacitance to the circuit which can impair the circuit frequency response or propagation delay.
It is therefore a further objective of this invention to produce a crossover structure which adds a minimum crosspoint capacitance to the circuit.